in reply to: Question about the AD421 DAC by Analog Devices posted by Eric Myers on 08 May 2002 at 14:30:09.
The AD421 is extremely sensitive to supply noise. This is noted on the data sheet as "must be well decoupled" and no noise suseptance spec is given. Decouple all the remainder of the circuit with a series 50 Ohm resistor and a cap on the side away from the AD421. This method is found on only one app.
that I could find. There are other methods, but this is probably the simplest/cheapest.